Can a heterosexual employee bring a harassment claim on the basis that they are offended by regular homophobic banter between colleagues?

Yes, such a claim is possible as the Equality Act 2010, which prohibits sexual orientation discrimination, is worded in such a way as to protect all employees from sexual orientation harassment, whatever their sexuality. Further, the provisions on harassment in the Act refer to behaviour that is "related to" sexual orientation and do not specify that any offence caused must be on the grounds of the individual's personal sexual orientation. This means that a heterosexual employee who finds homophobic banter at work offensive may legitimately bring a complaint to an employment tribunal on the basis that such conduct creates an intimidating, hostile, degrading, humiliating or offensive environment for them, irrespective of the individual's sexual orientation.